1. Field of the Invention
The present invention relates in general to operational control of an air conditioner, more particularly, to a method for controlling an air conditioner, in which sucking/discharge compressure of a compressor of an outdoor unit of the air conditioner currently being run are compared to predetermined sucking/discharge compressure to see whether or not the driving mode of the compressor is in a safe or unstable area, and if it turns out that the driving mode of the compressor is unstable variable factors are controlled to make the driving mode of the compressor be in a safe area.
2. Discussion of the Background Art
FIG. 1 is a cross-sectional view of an indoor unit and an outdoor unit of a related art separate-type air conditioner.
As shown in FIG. 1, the separate-type air conditioner includes the outdoor unit 1 installed outside and the indoor unit 2 installed indoors.
The outdoor unit 1 includes a compressor 10 for compressing a refrigerant to a high-temperature/high-pressure state, a condenser 12 for changing the refrigerant from the compressor 10 to a room-temperature/high-pressure state, an outdoor fan 13 for cooling the condenser 12, and a fan motor 5.
The indoor unit 2 includes an EEV (electrical expansion valve) 14a for expanding the refrigerant provided from the condenser 12 to a low-pressure state, an evaporator 15 for changing the refrigerant from the EEV 14a to a low-temperature/low-pressure state, and a fan 16 for blowing indoor air sucked into the indoor unit 2 at the evaporator 15 for heat change by force.
The location of the EEV 14a in the indoor unit 2 of FIG. 1 is only illustrative, that is, the EEV 14a can also be installed at the outdoor unit 1.
FIG. 2 is a cross-sectional view of an EEV 14a installed at the outdoor unit.
The EEV 14a is a device for expanding the room-temperature/high-pressure refrigerant from the condenser 12 (see FIG. 1).
As depicted in FIG. 2, the EEV 14a circulates the refrigerant through an inlet pipe 22a and an outlet pipe 23a. 
The refrigerant from the condenser 12 flows in the EEV 14a via the inlet pipe 22, 22a. When a needle 3 translates in the vertical direction by an operation of a screw 8 induced by a stepping motor 9, an orifice 4 causes the refrigerant to expand to the outlet pipe 23, 23a and to be injected.
Therefore, the injection flow of the refrigerant is controlled by the needle 3 and the orifice 4.
FIG. 3 is a cross-sectional view of the outdoor unit 1 including the EEV 14a, and FIG. 4 is a cross-sectional view of the indoor unit 2 including the EEV 14a. 
Referring first to FIG. 3, the room-temperature/high-pressure refrigerant from the condenser 12 flows in the EEV 14a through the inlet pipe 22, 22a and is expanded and eventually led to the outlet pipe 23, 23a. 
Referring now to FIG. 4, the room-temperature/high-pressure refrigerant from the condenser 12 of the outdoor unit 1 flows in the EEV 14a through the inlet pipe 22, 22a and is expanded and eventually led to the outlet pipe 23, 23a. 
In general, an air conditioner is an apparatus for cooling air within a defined space by discharging cooled air to the inside. To generate the cooled air, a refrigerant undergoes compressing, condensing, expansion, and evaporation. As described before, the air conditioner includes an outdoor unit installed outside and an indoor unit installed indoors. The outdoor unit includes a compressor 10, an accumulator 17, a four-way valve 11, a condenser 12 functioning as an outdoor heat exchanger, and an outdoor fan 13. The indoor unit includes an expansion valve 14, an evaporator 15 functioning as an indoor heat exchanger, and an indoor fan 16.
The air conditioner may include one indoor unit and one outdoor unit. However in some case, a multi-air conditioner in which a plurality of indoor units is connected to one outdoor unit is preferred for cooling or heating many indoor spaces at a time.
FIG. 5 illustrates how the air conditioner with the above construction works. As shown in FIG. 5, low-temperature, low-pressure vaporized refrigerant flows into the compressor 10 from the indoor heat exchanger 15 and turns to high-temperature, high-pressure vaporized refrigerant under the compression of the compressor 10 and at the same time it is discharged to the outdoor heat exchanger 12 through the four-way valve 11 that is switched for a cooling cycle. The discharged refrigerant to the outdoor heat exchanger 12 circulates within the outdoor heat exchanger 12 and when the outdoor fan 13 operates, heat exchange is made between the refrigerant and outdoor air sucked into the outdoor unit. As a result, the high-temperature, high-pressure vaporized refrigerant undergoes a phase change to the room temperature, high-pressure liquid refrigerant.
The liquid refrigerant is then discharged to the expansion valve 14 where the liquid refrigerant is decompressed to a low-temperature, low-pressure liquid state to be evaporated more easily, and the decompressed refrigerant is discharged to the indoor heat exchanger 15, where the refrigerant does heat exchange with the ambient air in the indoor heat exchanger 15 to be low-temperature, low-pressure vaporized refrigerant. This low-temperature, low-pressure vaporized refrigerant flows in the compressor 10 again through the four-way valve 11.
Therefore, the indoor air that was involved in heat exchange with the decompressed refrigerant through the expansion valve 14 in the indoor heat exchanger 15 loses the heat to the refrigerant and turns to cooled air, and this cooled air is discharged into the inside through the indoor fan 16, resulting in the cool comfort of indoor air conditioning.
The heating process of the air conditioner is just opposite to the cooling process thereof. As FIG. 6 illustrates, low-temperature, low pressure vaporized refrigerant from the outdoor heat exchanger 12 is compressed by the compressor 10 and turns to high-temperature, high-pressure vaporized state. This high-temperature, high-pressure vaporized refrigerant is discharged to the indoor heat exchanger 15 through the four-way valve 11 that is switched for a heating cycle. In the indoor heat exchanger 15 is made heat exchange between the discharged vaporized refrigerant and the ambient air of the indoor heat exchanger 15 so that the high-temperature, high-pressure vaporized refrigerant experiences a phase change to room-temperature, high-pressure liquid state and is discharged to the expansion valve 14. Here, the ambient air that was involved in heat exchange with the high-temperature, high-pressure vaporized refrigerant turns to hot air by taking heat out of the refrigerant and at the same time, is discharged to the indoors through the indoor fan 16, resulting in the increase of indoor temperature.
Further, the refrigerant discharged to the expansion valve 14 is decompressed to low-temperature, low-pressure liquid state and discharged to the outdoor heat exchanger 12 for evaporation. In the outdoor heat exchanger 12 is heat exchange made between the refrigerant discharged to the outdoor heat exchanger 12 and outdoor air flown in the outdoor unit. After heat exchange, the low-temperature, low-pressure liquid refrigerant turns to low-temperature, low-pressure vaporized state and flows in the compressor 10 again through the four-way valve 11.
The above-described cooling and heating processes are possible by controlling desired pressure and temperature.
In other words, the related art air conditioner runs in accordance with a control system of sucking/discharge compressure of the compressor 10 or a predetermined driving step of the compressor 10 obtained by calculating indoor/outdoor temperatures and the capacity of the indoor unit to be operated. When the air conditioner runs on the basis of the compressure and temperature values, at least one of the sucking compressure and discharge compressure of the compressure 10 in operation is lowered. In such case, the compressor 10 departs from a safe area where vibrations/noises of the compressor 10 are low and the compressor 10 does not overrun, and enters to an unstable area as shown in FIG. 7, in which vibrations/noises of the compressor 10 are great.
When the compressor 10 runs in the unstable area for an extended period of time, the compressor 10 is easily damaged by the increase of vibrations/noises and needs to be replaced soon. Mechanical damages on the compressor 10 consequently deteriorate performance and capacity of the air conditioner.
Moreover, if the compressor 10 is damaged primarily because of the increased vibrations/noises, it brings a serious problem to the reliability and durability of the compressor itself.